There are generally two basic types of smoke detectors commonly used today in working and living environments, such as buildings and homes. They are ionization and photoelectric type smoke detectors. The ionization type detector is the most prevalent type of smoke detector and is most responsive to flaming fires. The photoelectric type detector is generally considered more responsive to smoldering fires, which are those that begin with a long period of smoke build-up but no flames. For the best protection, the National Fire Protection Association recommends that both types be installed.
For smoke detection, a combustion aerosol commonly called smoke is composed of particles ranging in size from below those that are visible to those that are readily visible under ordinary light and are what smoke detectors detect. This applies to both the ionization-type smoke detector as well as the photoelectric-type smoke detector. Therefore, both types of smoke detectors can be thought of as particle detectors or, to be more precise, as particle counters.
Depending on the alarm response set point, smoke detectors produce an alarm when a certain number of particles are present within their sensing chambers. This particle-counting ability of the smoke detectors is complicated by the fact that the particle diameter also influences their response. In the case of the photoelectric smoke detectors, the wavelength of light used in the detectors as well as the complex index of refraction of the smoke particles also influences their response.
Statistics published by the U.S. Federal Emergency Management Agency in 2005 show that fires caused by smoking and electrical malfunction account for 29.5% of all deaths, despite only accounting for 10.3% of all fires. Cooking on the other hand, accounts for 39.9% of all fires, but only 5.1% of deaths. Apparently, it is fires that occur when no one is watching or nearby that are the most dangerous. The most prevalent flammable fibers in a home or building are cotton, rayon, nylon, acrylic, polyester, polylactide (PLA), olefin, and polyurethane, while the most prevalent wiring insulation materials are polyvinyl chloride (PVC), polyethylene, rubber, polychloroprene, and TEFLON®. As these materials are heated, melting and/or smoldering thereof eventually creates smoke particles that conventional smoke detectors can detect. However, long before a situation reaches that stage, materials are being heated and evaporated. As the temperature increases, the heated substances begin to vaporize and also decompose, giving off vapors that spread throughout a room or building. If these vapors could be reliably detected before smoke particles are generated, an early warning could undoubtedly save many lives. Indeed, it would be extremely useful to sense or detect and warn of smoldering fabric or electrical insulation before smoke is generated.
With respect to fires or potential fires, materials composed of polymers decompose via a series of chemical reaction mechanisms when heated to a sufficiently high temperature. The main mechanisms that reduce molecular weight of the polymers are random chain scission, chain-end scission (unzipping), and chain stripping (removal of side groups). Two other thermally induced processes, cross-linking and condensation, have the opposite effect of increasing molecular weight. Although decomposition often involves more than one of the scission mechanisms, the dominant reaction in most polymer systems is random chain scission. This commences with the weakest bonds in the chain, which is usually where an irregularity occurs in the molecular structure due, for instance, to the presence of a tertiary carbon atom, as in polypropylene, or other relatively unstable linkages with low dissociation energies. Scission usually proceeds randomly throughout the length of the chain.
With increasing temperature, other chemical bonds with higher dissociation energies rupture, causing the resulting segments to breakdown further into monomers, oligomers (i.e., polymer units with ten or fewer monomer units), and other low molecular weight species. It is noteworthy that while random chain scission can decompose long polymer chains into an extremely large number of fragments, in general, only a few percent of the bonds need to rupture to drastically degrade the mechanical properties. A bond rupture level of about 10% is generally sufficient to generate organic compounds that are volatile in a fire. For fragments to be small enough to diffuse through the polymer char into the fire, fragments molecular weight must be lower than about 400, although with many volatile species the molecular weight (MW) is much less (for example, styrene MW=96). It is these volatiles that decompose at the fire/composite interface that produce heat that sustains the decomposition process.
Chain-end scission (unzipping or depolymerization) is another important decomposition reaction that can compete with random scission in some polymer systems. Here, individual monomer units or volatile chain fragments are successively removed at the chain end until the polymer molecule has completely depolymerized. Chain stripping is a further decomposition reaction that involves the removal of side groups.
Cross-linking results, often temporarily, in an increase in molecular weight, in competition with the processes mentioned above. In most thermosets, for instance, it is well-known that “post-cure” or further cross-linking occurs at elevated temperature (say above 100° C.-150° C.) and precedes the decomposition processes that occur at higher temperatures (typically above 250° C.-400° C.). Likewise, in some thermoplastics (e.g., polyethylene), a degree of cross-linking precedes chain scission.
The thermal decomposition reactions of polymers may proceed by oxidative processes or simply by the action of heat. The decomposition process is often accelerated by oxygen, but in thick composite sections it is generally only the surface region that decomposes in the presence of oxygen. The out-gassing of volatiles from the decomposition zone impedes the ability of oxygen to diffuse much beyond the surface layers of the composite. Therefore, atmospheric oxygen does not have a major influence as the decomposition process moves deep into thick section composites, where decomposition tends to be driven mainly by heat.
Some polymers undergo random chain scission, end-chain scission, and chain stripping reactions, which leads to the loss of hydrogen atoms, pendant groups, and other low molecular weight organic groups from the main chain. These polymers yield a small amount of char (typically 5-20% of the original mass) and they include polyesters, vinyl esters, epoxies and polyvinyl chlorides (PVC).
Yet other polymers are characterized by a high aromatic ring content that decomposes into aromatic fragments that fuse via condensation reactions to produce moderate to high amounts of char. Aromatic rings are the basic building blocks from which char is formed, and, therefore, the higher the aromatic content of the polymer, the higher the char yield. Char yield increases linearly with the concentration of multiple-bonded aromatic ring groups in the polymer system. These aromatic groups are transformed at high temperature into pitch-like entities that eventually combine into char. A well-known polymer from the viewpoint of char formation is phenolic, in which 40-60% of the resin mass is converted to char. Several other polymer systems yield high amounts of char, and these include highly aromatic thermosets (e.g., polyimides, phthalonitriles, epoxy novolacs, and cyanate esters) and certain thermoplastics (e.g., polyphenylene sulfide (PPS), poly(p-phenylene oxide) (PPO), and polyether ether ketone (PEEK)).
Different chemical compounds result from the pyrolysis of household materials depending on the temperature at which they decompose. For instance, high-temperature pyrolysis of neoprene results in a substantial increase in the production of the following hydrocarbons: ethylene, acetylene, ethane, propane, propylene, propyne, n-butane, I-butene, isobutylene, cis-2-butene, trans-2-butene, ethylacetylene, and pentane. The main difference between these high-temperature degradation products and those from medium-temperature pyrolysis is a reduction in the quantity of liquid pyrolyzates, particularly in the lower-molecular-weight compounds. Pyrolysis at medium temperatures for rubber-134 show mainly the production of higher molecular weight aliphatic hydrocarbons, aliphatic alcohols, naphthalene, benzoic, and phthalic acids. Those from rubber-138 are predominantly aromatic.
A device and system that can detect pre-smoke vapors resulting from the vaporization of common household substances or the resultant breakdown, decomposition, or pyrolysis of these substances, could be a vital addition to the normal compliment of smoke detectors that are typically used to warn of fires or potential fires. In the case of fires that result from smoldering while the occupants are asleep, such as fires initiated by electrical malfunction or smoking, a significant decrease in the occurrence rate of both fires and fatalities might be the desirable result.
Accordingly, there is a need for a pre-smoke device or detector that can reliably identify individual analytes that are indicative of a pre-smoke condition, or alternately reliably detect key analytes as a class depending on their characteristics, for use in the early detection and warning of fires or potential fires.